This is a topic I am quite passionate about as many SEO firms in my opinions try and deceive the actual truth.
The rule is that each page regardless if it is the main page or subpage should cater for no more than 5 keywords, out of those 5 keywords it is likely that you will rank 3 of them quite highly assuming you apply the proper SEO techniques.
For your homepage you want to target the big keywords that will drive the most traffic to your site and for the subpages target the niche keywords relevant to those keywords.
When we take a new client on we always 1st start the SEO campaign with 5 keywords which are chosen from analytics, keyword research etc. The 1st few months of the SEO campaign we spend 80% of our time getting the homepage ranked high, until your homepage is ranked high their is little chance of getting your subpages to rank high.
Now once the homepage is ranking nicely we will then target all the keywords relevant to your subpages in the desire to rank your subpages nice and high as well.
So while many people may assume we only target 5 keywords this is incorrect, it is 5 keywords per page with the homepage gaining the most attention in the beginning few months of the commencment of the SEO campaign.
